1. Field of the Invention
The present invention relates to a method of manufacturing a rotor magnet for a micro rotary electric machine, and more particularly to a method of manufacturing a rotor magnet including a predetermined number of thick films layered on one another, which include magnetically isotropic nano-crystalline texture composed of a high saturation magnetized αFe phase and a high coercive R2TM14B phase, and which are multi-polar magnetized in-plane, to thereby provide a radial gap type micro rotary electric machine adapted to improve a high mechanical output power.
2. Description of the Related Art
A rotary electric machine for application field, for example, information and telecommunication devices has been commercially produced with its volume reduced to about 100 mm3 and is widely used. Such a rotary electric machine, when further miniaturized successfully, is expected to create a market for a motor adapted to output dynamic power or a generator adapted to produce electric power as a magnetic device in the so-called Power MEMS (Power micro-electromechanical system) field for new era.
Patent Document 1, for example, discloses a radial gap type brushless DC motor (RG-BLM) with an outer diameter of 1 mm or less and an axial length of 2 mm or less, which includes a hollow circular cylinder having a conductive cylindrical wall with slots and functioning as an excitation winding, and which is applied to an intravascular ultrasonography system. Also, Patent Document 2 discloses a fluid-cooled RG-BLM which has an outer diameter of 8 mm or less, and accordingly can be introduced into a vascular system of a body to thereby drive a blood pump located in the body, and in which an excitation winding is molded by resin containing Al2O3 thereby enhancing heat dissipation performance thus enabling achievement of an output of 5 W at 30,000 rpm.
As an example of a micro rotary electric machine as described above, a brushless DC motor is known which has a volume of 4 mm3 with an outer diameter of 1.6 mm and an axial length of 2 mm wherein a one pole-pair rotor having an outer diameter of 0.76 mm and including an Nd2Fe14B sintered magnet produced by electric discharge machining is coupled to a stator (refer to Non-Patent Document 1). Also known are a brushless DC motor with a volume of 62 mm3 (an outer diameter of 6 mm and an axial length of 2.2 mm) proposed by H. Raisigel (refer to Non-Patent Document 2), further a brushless DC motor with a volume of 20 mm3 (an outer diameter of 5 mm and an axial length of 1 mm) proposed by M. Nakano (refer to Non-Patent Document 3), and still further a brushless DC motor with a volume of 0.6 mm3 (an outer diameter of 0.8 mm and an axial length of 1.2 mm) proposed by T. Ito (refer to Non-Patent Document 4). The micro rotary electric machines described above undergo a significant decrease in torque due to the volume reduction according to the scaling law. However, since the mechanical output power P (W) is obtained by a product of: a constant number k=0.1047 (=π30); a revolution number N (r.p.m.); and a torque T (Nm), the decrease of the output P of a rotary electric machine resulting from the volume reduction can be supplemented to some extent by an increased rotational speed.
Various proposals have been presented for rotor magnets for use in the micro rotary electric machines as described above. For example, D. Hinz, et al. introduce an Nd2Fe14B system magnet with a thickness of 300 μm, which is die-upset at 750° C. and has a remanence, Mr,=1.25 T, a coercivity, HcJ,=1.06 MA/m, and a (BH)max=290 kJ/m3 (refer to Non-Patent Document 5). Also, J. Delamare, et al. represent that a torque of 0.001 mNm is generated by a motor which includes a rotor with an SmCo system magnet having eight pole-pairs and a stator disposed to oppose the rotor and which is driven at 100,000 rpm, or that an electric power of 1 W is produced by an electric generator which includes the above described rotor and stator and which is driven at 150,000 rpm (refer to Non-Patent Document 6). Further, Toepfer, T. Speliois, et al. report a so-called Power MEMS motor including a rotor with an Nd2Fe14B bonded magnet which is screen-printed on an FeSi substrate with a diameter of 10 mm so as to have a thickness of 500 μm, and which has a remanence, Mr,=0.42 T, and a (BH)max=15.8 kJ/m3 (refer to Non-Patent Document 7). In terms of torque per volume of a micro rotary electric machine, a radial gap type has an advantage over an axial gap type (refer to Non-Patent Document 8). Accordingly, a rotor magnet must be multi-polar magnetized in the radial direction to thereby fully derive the magnetic potential inherently present in a material.
A general two-pole permanent magnetic field of, for example, a DC motor can be pulse-current magnetized using a solenoid coil so as to achieve a magnetizing field of 4 MA/m or more. In this connection, in order to enable a micro rotary electric machine to achieve a higher dense torque, it is preferred to increase the number of magnetic poles as seen in the eight pole-pair rotor of J. Delamare, et al (refer to Non-Patent Document 6). When a rotor magnet is designed to have four or more poles with an inter-pole distance of about 1.5 mm, it is usual to apply pulse-current magnetization using a magnetization yoke of 1 turn/coil. However, considering the durability of the magnetization yoke (conductor), the peak value of pulsed current, IP, is set at a current density of up to about 25 kA/mm2 for the conductor. That is to say, if the inter-pole distance decreases, the conductor diameter is decreased, and consequently the peak value of pulsed current, IP, allowed is lowered. As a result, when the rotor magnet is further miniaturized and magnetized with an increased number of pole-pairs, saturation magnetization is inevitable thus making it difficult to fully derive the magnetic potential inherently present in a material.
H. Komura, et al. disclose a fine magnetization of a rotor magnet for use in such a micro rotary electric machine as described above, wherein a bonded magnet with an outer diameter of 2.6 mm, which includes an isotropic Nd2Fe14B magnetic powder (Currie temperature Tc=320° C.) prepared from a melt-spinning and cured with an epoxy resin, and which has a remanence, Mr,=about 0.7 T, is put in a magnetic field system fabricated from a 2-17 type SmCo system sintered magnet, is heated up to Tc (320° C.) or higher, and then cooled in the magnetic field system, whereby the rotor magnet is magnetized with eight pole-pairs. The magnetization described above is said to have achieved a magnetic flux almost three times as high as the magnetic flux achieved by the usual pulse-current magnetization of 1 turn/coil (refer to Non-Patent Document 9).
As for a magnet which includes an isotropic nano-crystalline texture according to the present invention composed of αFe and R2TM14Bs, usually an R-TM-B molten alloy is melt-spun making an amorphous thin ribbon of αFe and R-TBM, and then the amorphous thin ribbon is treated with heat and is thereby crystallized. Material obtained by rapid solidification of a molten alloy is limited to a powdery substance, such as a melt-spun thin ribbon having a thickness of about 15 to 40 μm. Accordingly, the powdery substance must be consolidated into a specific bulk by some means in order to be used as a magnet. Such consolidation of the powdery substance can be achieved, as disclosed by Toepfer, et al., T. Speliotis et al. and H. Komura, et al., mainly by mixing with a binder, for example, an epoxy resin, whereby a bonded magnet is formed.
Reference Documents which have so far been cited and/or will hereinafter be cited are listed as follows:    <Patent Document 1>PCT Patent Application Laid-Open No. H9-501820    <Patent Document 2>Japanese Patent Application Laid-Open No. 2002-532047    <Patent Document 3>Japanese Patent Application Laid-Open No. H9-23771    <Patent Document 4>Japanese Patent Application Laid-Open No. H11-288812    <Non-Patent Document 1>Mitsubishi Electric Corp. Technical Report—Volume 75 (2001), pp. 703-708, by S. Ohta, T. Obara, Y. Toda and M. Takeda    <Non-Patent Document 2>Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 942-944, by H. Raisigel, O. Wiss, N. Achotte, O. Cugat and J. Delamare    <Non-Patent Document 3>Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 723-726, by M. Nakano, S, Sato, R. Kato, H. Fukunaga, F. Yamashita, S. Hoefinger and J. Fidler    <Non-Patent Document 4>Journal of the Magnetics Society of Japan—Volume 18 (1994), pp. 922-927, by T. Ito    <Non-Patent Document 5>Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 76-83, by D. Hinz, O. Gutfleisch and K. H. Muller    <Non-Patent Document 6>Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 767-778, by J. Delamare, G. Reyne and O. Cugat    <Non-Patent Document 7>Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 942-944, by Toepfer, B. Pawlowski, D. Scha and B. Bel    <Non-Patent Document 8>Materials for the 143rd Workshop of the Applied Magnetics Society of Japan, Surugadai Kinenkan of Chuo University (2005), by F. Yamashita    <Non-Patent Document 9>Journal of Applied Physics—Volume 101 (2007), 09K104, by H. Komura, M. Kitaoka, T. Kiyomiya and Y Matsuo
With respect to the output of a micro rotary electric machine, a torque decrease due to volume reduction can be effectively supplemented by increasing rotational speed. However, an anisotropic magnet of, for example, D. Hinz, which has a remanence, Mr, of 1.2 T and an energy density (BH)max of 290 kJ/m3 (refer to Non-Patent Document 5), can be radially magnetized with a level of remanence, Mr, maintained in a limited case of magnetization for one pole-pair, but can hardly be oriented in a case of radial multi-polar magnetization for four or more poles thus making it virtually impossible to produce. Consequently, the above magnet can hardly be applied to a radial gap type rotary electric machine which is favorable in terms of torque density or output characteristics and, in a case of multi-polar magnetization with two or more pole-pairs, can be used only as a rotor magnet for an axial gap type rotary electric machine especially.
The anisotropic magnet described above is favorable for increasing torque for an axial gap type rotary electric machine, but unfavorable for increasing rotational speed due to S-T (speed-torque) drooping characteristic. Besides, the magnet has an electric specific resistance of 10-5 Ωcm or less, and therefore loss due to eddy current increases even if rotational speed is increased. Such loss due to eddy current turns into heat energy thereby raising the temperature of the magnet, which results in decreasing static magnetic field (flux loss). Accordingly, an anisotropic rare earth magnet produced by die-upsetting or sintering, which is known to provide a high energy density (BH)max value, causes an increase in loss and a decrease in mechanical output power especially when used as a rotor magnet for a radial gap type micro rotary electric machine for a high speed rotation thus synergistically lowering the efficiency of the micro rotary electric machine. As described above, a well-known anisotropic magnet having a high (BH)max value is not optimum especially in terms of output and efficiency of a radial gap type micro rotary electric machine, which restricts the structural and electrical design freedom of the micro rotary electric machine.
In view of the above anisotropic magnet with a high remanence and a high (BH)max, Toepfer, T. Speliois, et al. propose a rotor with a bonded magnet which has an electric specific resistance of 10−1 Ωcm and a remanence, Mr, of 0.42 T, and thereby intend to restrain eddy current for the purpose of increasing rotational speed (refer to Non-Patent Documents 7 and 8). The remanence, Mr, of about 0.42 T, however, generates too low a static magnetic field to be used for a rotor magnet of a micro rotary electric machine, which results in insufficient torque.
On the other hand, with regard to multi-polar magnetizing a magnet for a micro rotor in the radial direction, for example, H. Komura, et al. report a multi-polar magnetization wherein a bonded magnet, which is made such that an isotropic Nd2Fe14B powder made from a melt-spun thin ribbon is cured with epoxy resin and which has a remanence, Mr, of about 0.62 to 0.68 T, is heated up to 320° C. or higher and then cooled in the magnetic field system (Non-Patent Document 9).
Since the above compression-molded bonded magnet of H. Komura, et al. has an electric specific resistance of about 102 Ωcm, the problem associated with eddy current can be avoided, but the magnetic powder as a base constituent as well as the epoxy resin are inevitably degraded by heat during the multi-polar magnetization thus causing the magnetic potential to drop. Further, the bonded magnet is deteriorated in mechanical strength, which raises a major problem with reliability in resistance to centrifugal force at a high speed rotation, and the like. Moreover, the bonded magnet with a remanence, Mr, of 0.62 to 0.68 T produces too weak a static magnetic field for use as a rotor magnet of a micro rotary electric machine like the examples of Toepfer, et al, and T. Speliotis, et al., thus failing to obtain sufficient torque.